An OLED (organic light emitting diode) display is a current-driven light emitting device. That is, the OLED display is driven to emit light by a current generated by a driving tube TFT in a saturated state.
No matter Low Temperature Poly-silicon Transistor (LTPS-TFT) or Oxide-TFT, non-uniformity during manufacturing always results in that transistors in different locations have a threshold voltage difference, which is vital for driving consistency of a current-driven device (such as an OLED light emitting element), because when threshold voltages of different driving tubes are different in the case of inputting the same gray-scale voltage, the different threshold voltages will produce different driving currents, and thereby lead to inconsistency of the driving currents. Therefore, the conventional OLED driving circuit needs to compensate for the threshold voltages of the driving tubes, so that the driving currents are no longer affected by inconsistency of the threshold voltages of the driving tubes.
In addition, as the use time of the OLED light emitting element increases, a great many of non-recombined carriers accumulate in an internal interface of a light emitting layer of the OLED light emitting element, and accumulation of the carriers causes a built-in electric field to be formed inside the OLED light emitting element, makes the threshold voltage of the OLED light emitting element rise, which directly causes aging of luminescent material of the OLED light emitting element and shortens a lifespan of the OLED light emitting element.
At present, the conventional OLED driving circuit requires at least seven transistors to simultaneously achieve compensation for the threshold voltages of the driving tubes and aging alleviation of luminescent material of the OLED light emitting element, which limits the OLED display's resolution to a certain extent.